1. Field of the Invention
The present invention relates to an acidic tinplating bath and an additive therefor.
2. Description of the Related Arts
Organic sulfonic acid bath is mainly used in conventional tinplating bath to apply electro-tinplating on a steel sheet for manufacturing cans. In particular, ferrostanate bath is widely used which uses PSA (phenol sulfonic acid) as the organic sulfonic acid and aromatic sulfonic acid such as ENSA (ethoxynaphthol sulfonic acid) as the brightening agent.
Ferrostanate bath has, however, a problem that the bath fails to provide surface brightness requested for tin can material and to provide adequate corrosion resistance after the reflow treatment (tin fusion heating) if the electrolytic condition falls in outside of adequate range. Accordingly, the operating range of the ferrostanate bath is limited to approximate electrolysis condition range from 20 to 30 A/dm.sup.2 even in a line speed range from 200 to 400 m/min., and the ferrostanate bath has problems of many limitation in operating conditions and of low productivity. In addition, ferrostanate bath needs many electrolytic cells to obtain a specified tin coating weight so that this type of bath is inferior in investment cost. Furthermore, ferrostanate bath has recently raised a problem of bad influence to environment.
To solve these problems of ferrostanate bath, various types of baths relating to the continuous tinplating of steel sheet have been introduced. Among them, a bath based on MSA (methane sulfonic acid), an alkanesulfonic acid, draws attention as a continuous electro-tinplating bath for steel sheet switching from the ferrostanate bath because the bath is suitable for high current density electrolysis owing to less electrolytic conductivity compared with the PSA-based ferrostanate bath and because the bath generates less bad influence to environment.
Performance of plating bath used for tinplating is significantly affected not only by the type of organic sulfonic acid as the basis but also the additive used in combination therewith. Consequently, many kinds of additives were proposed also for a bath that uses MSA (methane sulfonic acid) as the basis.
Nevertheless, most of these proposed additives are used in tin-lead alloy plating such as solder plating and bright electroplating in electronics devices. As for the additives in matte electrodeposition used for continuous tinplating process of steel sheet, not many proposals were made, and only the following disclosures have been given.
Japanese unexamined patent publication No.4-228595 discloses an additive used in a continuous electro-tinplating process of steel sheet. According to the disclosure, the additive is an alkylene oxide condensation compound of an aromatic hydrocarbon, or an alkylene oxide condensation compound of an aliphatic compound having at least one hydroxyl group and having 20 or less of carbon atoms and containing 20 moles or less of oxide. The disclosure describes the evaluation on the performance of additive using a Hull cell under the electrolytic condition of 2 Ampere and low flow environment to determine post-electroplating appearance, cloud point and foamability. However, the disclosure does not give quantitative description on the evaluation other than the cloud point, and no quantitative suitability in a high speed and high current density operation is shown.
Japanese Unexamined Japanese patent publication No.3-291393 discloses a compound prepared by adding 20 moles or less of ethylene oxide and propylene oxide to .alpha.-naphthol and sulfonating it as an additive used in a continuous electro-tinplating process of steel sheet. The use of the additive aims to operate the process under a high speed and high current density condition. The disclosure gives evaluation of uniform electrodeposition, solubility of additive, and foaming property after the electrolysis at 50 A/dm.sup.2 in high flow environment. The disclosure, however, does not give any evaluation on post-electrolysis appearance which is an important variable of post-tinplating quality.
Based on the above-described prior arts, the inventors conducted investigation and study on additives which are applicable to tinplating bath of alkanesulfonic acid basis and which provide a wide range of current density and an increased upper limit of current density.
The evaluation of additives was given through the bath-preparation and plating under a constant condition of basis bath composition except for the additives and under a constant plating condition except for the current density, and through the evaluation of performance of the electrolytic characteristics and the evaluation of performance of the obtained plated steel sheet (plated deposit). The performance evaluation of the plated steel sheet was conducted mainly on the post-reflow brightness which is an important quality item of the tinplated steel sheet used for can material.
The result revealed that the brightness obtained in laboratory reflow treatment gives poor reproducibility owing to the influence of kinds and quantity of fluxes used in the reflow treatment, to the wettability of specimens, and to the heating pattern in the reflow treatment, and that accurate quantitative evaluation is difficult to derive.
Particularly in a tinplating bath of alkanesulfonic acid basis, the morphologic change of plated deposit against the increase in the plating current density differed from that in existed ferrostanate bath. That is, the increase in current density induced gradual change of crystal structure from fine and dense submicron crystals to coarse crystals of 1 .mu.m or larger size and further to dendrite. It was also found that the coarse crystal structure which is not observed in ferrostanate bath changed responding to the composition of the plating bath and to the kind of additive applied, and that post-reflow brightness was attained in some cases and could not be attained in other cases. However, the evaluation of brightness after the reflow treatment could not be performed from the structure of plated deposit before the reflow treatment. Furthermore, change in electrolysis behavior and change in brightness before reflow treatment against the plating current density did not agree with the change in crystal structure of coarse grains nor with the post-reflow brightness.
Accordingly, the tinplating bath of alkanesulfonic acid basis is difficult to give relative evaluation of additive performance because there is no accurate method for evaluating bath performance including post-reflow brightness. Thus there existed a serious problem of difficulty in developing and selecting additives for tinplating bath having excellent characteristics of high current density electrolysis.